Modeling and control of a pneumatic artificial muscle manipulator joint – Part I: Modeling of a pneumatic artificial muscle manipulator joint with accounting for creep effect

An antagonistically actuated pair of pneumatic muscle actuators has recently become an interesting basic joint unit for developing humanoid robots or robotic manipulators in general. This basic unit exhibits some advantageous characteristics, such as high compliance, high power-weight ratio, and high volume-to-weight ratio. However, hysteresis and muscle creep are significant drawbacks that limit the performance of this manipulator unit.This paper presents a new approach to model the hysteresis, also taking into account the muscle creep, of a basic antagonistic manipulator joint constructed by a pair of Festo fluidic muscles. The experimental results show that the manipulator hysteresis has the same behavior as that found in the individual muscles. This behavior is well described by the Maxwell-slip model. The creep effect is factorized and incorporated into the constraint torque model, resulting in a stationary extracted hysteresis loop. The time-varying behavior, due to creep, is transformed into a time-invariant one, and only one identified set of Maxwell-slip model parameters is proven to be adequate. This model performs well in the prediction of the torque–angle hysteresis, not only with an arbitrary angular trajectory at any isobaric condition, but also at any moment in the creep history of the unit.